In microwave-assisted chemistry, microwave energy is used to increase temperature in chemical synthesis, chemical analysis and similar processes. Thereby, known chemical reactions and processes can be accelerated, the yield can be increased, and the purity of the products can be improved. However, the large amount of energy, which is applied to the samples in microwave-assisted chemistry, also enables completely new syntheses and reactions.
Many devices and methods for microwave-assisted chemistry are based on household microwave ovens and operate at frequencies around 915 MHz or 2.45 GHz. Standing waves of different longitudinal and transversal modes of the microwave field are super-positioned within the cavity of such ovens. Therefore, the spatial energy distribution is non-homogeneous and exhibits so-called “hot spots” and “cold spots”. Examples of such classical multi-mode microwave heating systems are e.g. Anton Paar “Multiwave 3000”, CEM Mars Express, and Milestone “Ethos”.
Microwave transparent reaction vessels are arranged in microwave transparent vessel carriers and positioned within the microwave oven which is flushed with microwaves. Due to the non-uniform energy distribution, it is customary to place the reaction vessels on a rotatable support structure, such as a turn table. By rotating the support structure during heating, the level of applied energy to the samples may be evened out. It is furthermore known to implement mechanisms for stirring the samples and to change the superpositioned modes within the cavity, i.e. so-called modemixing. Single mode devices may be used for small sample amounts.
The large pressure and temperature that may occur in the reaction vessels during sample heating in a multi-mode device require the use of expensive materials for the interior structure of the heating devices which can resist the impact of a sample leaking from a broken vessel. Installation of temperature and pressure sensors is a complicated task due to the need for protecting such sensors against microwave radiation. Furthermore, care has to be taken as regards material and geometry of the sensor components in order to assure that these do not heat up and/or cause sparks when subjected to microwave radiation.
Also the reaction vessels are designed to withstand the pressure and temperature occurring during sample heating. The reaction vessels are thermally decoupled from the microwave transparent vessel carriers. Thus, a non-uniform microwave heating of each sample may occur, which may cause insufficient temperature homogeneity in the samples and thereby insufficient reproducibility of the conditions within the particular vessels.
In microwave reactors, microwave transparent reaction vessels (supported by microwave transparent sample carriers or rotors comprising separate carrier positions) are positioned in a metallic pressure-container which is then pressurized with nitrogen. This causes sealing of the reaction vessels which are provided with loose plugs. Thereafter, microwave is fed to the container. Thus, the cavity of a microwave reactor must be gas-proof, and a nitrogen source must be provided. Further, air cooling is inefficient and automatic ventilation is not possible. Due to the high nitrogen consumption and the need for cost-effectiveness, the volume of microwave reactor cavities tends to be kept down, such that the number and size of sample vessels which fit into the reactor is limited.
The samples which are to be heated may have different permittivities and the permittivities may also change during heating. Differing filling levels of the inserted vessels may further modify the microwave propagation conditions within the cavity and thereby change the energy distribution within the cavity. WO 99/17588 A1 describes a complex way of dealing with this problem. Single-mode systems with only a single sample vessel carrier are described in US 2004/0069776 A1 and EP 2 244 529 A1. Systems with individual feeding of microwave to multiple sample carriers are described in WO 00/36880 A2 and in WO 2011/097116 A1.
There may be a need for a simple and inexpensive microwave-assisted chemistry heating system without the above-mentioned drawbacks. In particular, there may be a need for a microwave heating system with reduced dead volume and improved homogeneity of sample heating.